PALM SPRINGS, Calif.—Editing a manuscript might not be the most cinematic endeavors about which to make a documentary—unless the parties in question are Robert Caro and Robert Gottlieb. For a half-century, Gottlieb has edited Caro’s work, including the not-yet-completed final volume of Caro’s definitive biography of Lyndon Johnson. “Turn Every Page — The Adventures of Robert Caro and